Trials carried out by New Zealand scientists have shown how changing pastures can directly reduce the emissions of methane - a potent greenhouse gas - from sheep and cattle.

The researchers say it all comes down to the tannins, the yellow-brown chemicals found in many plants.

The scientists at New Zealand's agricultural research institute, AgResearch Grasslands, tested the legume lotus and found that its natural condensed tannin compounds reduced the methane emissions from ruminant animals by as much as 16%.

The country is keen to find ways of reducing the impact of its belching sheep and cattle so that it can meet targets for reducing those gases thought to be accelerating the warming of the planet.

Direct effects

The role of plants with condensed tannins in boosting productivity and lowering methane emissions has been known for some time, said Dr Julian Lee, from AgResearch Grasslands.

Tannins are found in many plants

"For a given amount of dry matter intake by a ruminant, as the nutritive value of the plants increases the amount of methane emitted per unit of productivity decreases. So, while productivity goes up, methane in relation to productivity as a proportion goes down. "

The missing knowledge, however, has been the understanding of the direct effects of the condensed tannins - diverse compounds, present in wine, apples and cocoa as well as some pasture species - separate from their nutritional effects.

"We've never been able to demonstrate up till now the direct effect of particular tannins," Dr Lee said.

Big wind

In the latest trials, AgResearch scientists, Drs Garry Waghorn and Michael Tavendale, were able to separate out the direct effects of particular condensed tannins from their nutritional effects in the legume lotus.

New Zealand may make changes to the way its animals graze

The AgResearch group studying methane will continue developing this research.

"We need to do a lot more work in understanding the type of tannin... how it works so that we can obviously maximise benefits from that," Dr Lee said.

Lowering New Zealand's methane emissions is necessary if the antipodean country is to meet its targets under the Kyoto Protocol, the international treaty that aims to reduce human influence on the global climate.

Tax trade-off

As a byproduct of their digestion, New Zealand's 45 million sheep and eight million or so cattle produce about 90% of the country's methane emissions.

And in total, they are responsible for an estimated 43% of all the country's greenhouse gases.

The average New Zealand dairy cow produces something like 90 kilograms of methane per year, equivalent in energy to 120 litres of gasoline.

The New Zealand Government says it plans to exempt its agricultural sector from any carbon taxes it imposes to ensure Kyoto targets are met.

However, in return, the agricultural industry has been told it must research ways to reduce agricultural emissions.